This invention relates to a method and apparatus for generating the pseudo noise code (PN code) employed for spread spectrum communication. More particularly, it relates to a PN code generating method and apparatus for detecting the timing of the PN code of the spread spectrum of the direct spread spectrum system in a receiver employed in a moving terminal of a so-called code division multiple access (CDMA) system digital Cellular and effecting an inverse operation for received signals.
In the spread spectrum communication system, the carrier wave is modulated (diffused) by pseudo noise (PN) code series at a transmitter side such as a base station and baseband demodulated to produce data after the correlation (inverse operation of spreading) process at a receiver such as a moving terminal using the PN code generated by the PN code generator having the same construction as that at the transmitter.
With the so-called CDMA system Cellular, which is a kind of the spread spectrum communication system, plural acceptable signals from among a number of signals transmitted from the same base station over multiple pass channels are utilized by way of bus diversity or RAKE reception, or plural signals transmitted form plural base stations are received simultaneously (soft hand off).
Since the base stations having high signal intensities are selected and received during soft hand off, an operation of changing over the base stations used for demodulation is frequently carried out within the receiver. Thus it is crucial to shorten the time which elapses since detection of the signal from a base station having a higher signal intensity than that of the currently received signal from another base station until actual reception of the higher intensity signal, that is the time necessary for switching from the signals of a base station to signals of another base station, in order to receive stronger signals more quickly and to enable them to be utilized for a longer time period.
In order to synchronize a PN sequence within a moving terminal to that in the base station, a PN sequence generator is advanced by being operated at a higher rate, or retarded by being operated at a lower rate or halted.
This method has however a drawback that a lot of time is required until synchronization is achieved. In the case of the CDMA system Cellular, the period of PN code is approximately 27 ms, so that a time period of 3.8 ms is required even if the rate of operation of the generator is increased by a factor of 7. For providing the 7-fold rate of operation, a number of clocks eight times as many as that for the PN code is necessitated. However, this rate cannot be increased infinitely because of the constraint imposed on the operating speed of various devices employed in the Cellular. There is also a fear that the power consumption is increased.